Field of the Invention
The present invention relates to the field of magnetometers. More specifically, the present invention relates to magnetometers based on spin ensembles, for example, alkali vapor cells.
Description of the Related Art
Magnetometers are used to measure the strength and direction of a magnetic field. They can be useful in measuring minute changes in the Earth's magnetic field that allow users to identify geological conditions under the Earth's surface, such as the location of oil and mineral deposits, as well as other conditions.
A magnetometer that uses a cesium atomic vapor is described in U.S. Pat. No. 7,723,985, issued May 25, 2010 to Kenneth R. Smith, which is incorporated by reference for all purposes. The basic principle that allows cesium atomic vapor magnetometers to operate is the fact that a cesium atom can exist in several energy levels, which involve the placement of electron atomic orbitals around the atomic nucleus. When a cesium atom within the cell encounters a photon from the emitter, it jumps to a higher energy state and then re-emits a photon and falls to an indeterminate lower energy state. The cesium atom is “sensitive” to the photons from the emitter only in certain energy states. Therefore, the atoms will preferentially populate those states that do not interact with the photons. The photons therefore pass through unhindered and are measured by the photon detector. Under this condition, the cesium in the cell is optically saturated.
Once the cesium is optically saturated, the system is ready for the measurement procedure. The axis of the atomic spin precesses about the ambient magnetic field. This precession causes the alignment between the atom and the light to vary, in a cyclic manner, between an alignment that favors the absorption of light and one that reduces the absorption. If the light is pulsed on and off at the same frequency as the precession rate of the atoms, those atoms that are aligned such that they absorb the light will be driven to a higher energy state. These atoms will then decay to one of the lower energy states. When the atoms return to the lower state, the phase of their precession will have been changed. If the precession is now such that the atom is aligned so as to not absorb light when the light pulses on, the atom will remain in this state. Thus, when the light is pulsed on and off at a rate equal to the precession rate, the absorption in the cell is decreased.
The wavelength of light from the emitter is typically modulated on and off of what is called an absorption line. This is the wavelength at which the absorption of the light in the cell is maximized. In Bell-Bloom systems, the modulation may be at a frequency known as the Larmor frequency. The Larmor frequency is the frequency of the atomic spin precession and is proportional to the strength of the magnetic field. A Bell-Bloom magnetometer typically tracks the variation in the Larmor frequency, which can be used to track the strength of the magnetic field, by slightly varying the rate at which the emitter is modulated and observing the variation in absorption with the frequency of the modulation.
The absorption line, however, is not a single line but a small group of sub-lines that are wider than their spacing so as to appear as a single line. The Larmor frequency for the different sub-lines is slightly different. These different Larmor frequencies form a combined Larmor frequency depending on the populations of the various lower energy states. As the emitter is modulated off the line towards states on one side, it tends to alter the populations of those states, and consequently alter the combined Larmor frequency. As a result, the details of the modulation affect the magnetic field measurement, leading to drift and heading error.
Furthermore, a semiconductor laser emitter does not respond to the modulation immediately, because of its thermal time constants. Attempts to modulate the light output at frequencies near the Larmor frequency (at the Earth's ambient field) are phase-shifted significantly by these time delays. These phase shifts complicate the operation of the magnetometer.